Types of smooth muscle

The two major types of smooth muscle (although many smooth muscles exhibit properties of each type) are:

• Multiunit smooth muscle

• Single-unit smooth muscle

Multiunit smooth muscle is located in the large blood vessels, eyes (iris and ciliary muscle of the lens), and piloerector muscles at the base of hair follicles. This type of muscle consists of discrete smooth muscle cells or units that function independently. Each of these units is innervated by the autonomic nervous system. In fact, like skeletal muscle, this type of smooth muscle must be stimulated by these nerves in order to initiate contraction. Therefore, this muscle is referred to as neurogenic. Interestingly, nerve stimulation elicits graded potentials only. Action potentials do not occur in this muscle. The amount of ion flux that occurs in a single muscle cell is inadequate to depolarize the muscle to threshold; however, the graded potentials are sufficient to cause smooth muscle contraction. The contractile response of the whole muscle results from the sum of the responses of multiple individual units.

Most smooth muscle is single-unit smooth muscle. Also referred to as visceral smooth muscle, it is found in the walls of tubes and hollow organs in the digestive, reproductive, and urinary systems, as well as in the walls of small blood vessels. The cells of this type of smooth muscle are connected electrically by gap junctions so that electrical activity can spread from one cell to the next, forming a functional syncytium. Any change in electrical activity in one region of the muscle quickly spreads throughout the muscle layer such that the cells of the muscle function as one, or as a "single unit."

Action potentials are generated in single-unit smooth muscle. Simultaneous depolarization of 30 to 40 smooth muscle cells is required to generate a propagated action potential; the presence of gap junctions allows this to occur readily. Because single-unit smooth muscle is self-excitable and capable of generating action potentials without input from the autonomic nervous system, it is referred to as myogenic. In this muscle, the function of the autonomic nervous system is to modify contractile activity only. Input is not needed to elicit contraction.

The ability to depolarize spontaneously is related to the unstable resting membrane potentials in single-unit smooth muscle. Two types of spontaneous depolarizations may occur:

• Pacemaker potentials

• Slow-wave potentials

A pacemaker potential involves gradual depolarization of the cell membrane to threshold. The subsequent generation of an action potential causes smooth muscle contraction. This type of spontaneous depolarization is referred to as a "pacemaker potential" because it creates a regular rhythm of contraction.

Slow-wave potentials also involve gradual depolarization of the cell membrane, but these depolarizations do not necessarily reach threshold. Therefore, the depolarization may simply be followed by repolarization back to the initial membrane potential. These slow "wave-like" potentials occur rhythmically and do not lead to smooth muscle contraction. The peak-to-peak amplitude of the slow-wave potential is in the range of 15 to 30 mV. Therefore, under the appropriate conditions, the depolarization phase of the slow-wave potential may, in fact, reach threshold. When this occurs, a burst of action potentials is generated, resulting in muscle contraction.

The mechanism of the slow-wave potential is unclear. One hypothesis is that the rate at which sodium ions are actively transported out of the cell rhythmically increases and decreases. A decrease in the outward movement of Na+ ions allows positive charges to accumulate along the internal surface of the cell membrane and depolarization takes place. This is followed by an increase in the outward movement of Na+ ions, which causes the internal surface of the cell membrane to become more negative, and repolarization takes place.

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